The measurement of testosterone levels is central to the diagnosis of androgen disorders in men and women. Circulating testosterone is bound largely to sex hormone binding globulin (SHBG) and albumin; only the unbound or free fraction is biologically active. Therefore, in conditions that affect SHBG concentrations, such as aging, obesity, and diabetes, the determination of free testosterone is necessary to obtain an accurate assessment of androgen status. The current methods for the determination of free testosterone - equilibrium dialysis, ultrafiltration, tracer analog methods, and the use of algorithms based on the law-of-mass-action - suffer from problems of imprecision or inaccuracy. Equilibrium dialysis method, widely considered the reference method, is labor-intensive and cumbersome, and susceptible to errors due to tracer impurities, leading the Endocrine Society's Expert Panel to conclude that ...the calculation of free testosterone is the most useful estimate of free testosterone in plasma... Therefore, algorithms for calculating free testosterone concentrations from total testosterone, SHBG and albumin concentrations using the law-of-mass-action equations have been used widely. Our preliminary data show that the current model of homogeneous testosterone binding sites on SHBG - the conceptual basis of the extant law-of-mass action equations - is erroneous, and that the free testosterone concentrations derived from the available law-of-mass action equations differ substantially from those measured by equilibrium dialysis.. Because of these limitations of the available methods, there is an enormous unmet need for an accurate method to determine free testosterone concentrations that can be applied conveniently in clinical practice. New evidence shows that testosterone's binding to SHBG conforms to an Ensemble Allostery Model (EAM). Based on this new EAM model of testosterone binding to SHBG, we have constructed a novel algorithm for calculating free testosterone levels that provides excellent conformity with values determined by equilibrium dialysis. This phase I SBIR application aims to characterize the technical performance of the novel algorithm by determining its precision, accuracy, functional sensitivity, intra- and inter-assay variation, and specificity, stability, and matrix effects (Aim 1). Aim 2 will incorporate SHB genotype in the algorithm and apply it to men with different SHBG genotypes. Aim 3 will characterize the performance of the assay in young and older men and women, and in obese men with alterations in SHBG concentrations. The product of the phase I studies will be a copyright-protected validated algorithm available as a stand-alone application that physicians can download on their personal computers or hand-held devices for use in their office or even on the bedside. An interdisciplinary team of investigators, strong preliminary data, and a validation plan that conforms to FDA guidance will assure the successful generation of a product for which there is considerable unmet need and which will advance our understanding of testosterone's binding to SHBG.